Quality of signal acquisition in networks is an ongoing concern in most networking environments. Much research is devoted to determining techniques for improving signal acquisition.
In network transmissions, Adaptive Multiresolution Modulation (AMM) permits adaptation of both the shape (β) and the size of an asymmetric signal constellation. The spectral efficiency gain achieved through the employment of AMM at the physical layer (of the Open Systems Interconnection model) may be substantial (e.g., up to fifty percent (50%) increase), especially at low and moderate carrier-to-noise ratio (CNR) (see, e.g., J. James et al., “Adaptive Multiresolution Modulation for Multimedia Traffic over Nakagami Fading Channels,” International Journal of Wireless & Mobile Networks (April 2012), pp. 1-20 (“James 1” hereinafter) and J. James et al., “Adaptive Multiresolution Modulation for Multimedia Traffic,” IEEE Consumer Communications and Networking Conference (Jan. 9-12, 2012), pp. 697-698 (“James 2” hereinafter)).
At the physical layer, there are three techniques that may facilitate Unequal Error Protection (UEP): 1) increase the transmission power while sending high priority bits; 2) use channel coding with varying levels of error protection; and/or 3) employ a suitable multiresolution (hierarchical) modulation scheme.
An example technique using UEP is discussed in K. Yang, et al., “Unequal Error Protection for Streaming Media Based on Rateless Codes,” IEEE Transactions on Computers, vol. 61 no. 5, pp. 666-675, May 2012 (“Yang” hereinafter).
As further explanation of constellations, a constellation diagram is a representation of a signal modulated by a digital modulation scheme such as quadrature amplitude modulation or phase-shift keying. The diagram represents the signal as a two-dimensional xy-plane scatter diagram in the complex plane at symbol sampling instants. The angle of a point, measured counterclockwise from the horizontal axis, represents the phase shift of the carrier wave from a reference phase. The distance of a point from the origin represents a measure of the amplitude or power of the signal.
In a digital modulation system, information may be transmitted as a series of samples, each occupying a uniform time slot. During each sample, the carrier wave has a constant amplitude and phase which may be restricted to one of a finite number of values, so each sample encodes one of a finite number of “symbols”, which in turn represent one or more binary digits (bits) of information. Each symbol may be encoded as a different combination of amplitude and phase of the carrier, so each symbol may be represented by a point on the constellation diagram. The constellation diagram may represent all the possible symbols that can be transmitted by the system as a collection of points. In a frequency or phase modulated signal, the signal amplitude is constant, so the points lie on a circle around the origin.
The carrier representing each symbol can be created by adding together different amounts of a cosine wave representing the “I” or in-phase carrier, and a sine wave, shifted by 90° from the I carrier called the “Q” or quadrature carrier. Thus, each symbol may be represented by a complex number, and the constellation diagram may be regarded as a complex plane, with the horizontal real axis representing the I component and the vertical imaginary axis representing the Q component. A coherent detector may independently demodulate these carriers. The principle of using two independently modulated carriers is the foundation of quadrature modulation. In pure phase modulation, the phase of the modulating symbol is the phase of the carrier itself.
A “signal space diagram” refers to an ideal constellation diagram showing the correct position of the point representing each symbol. After passing through a communication channel, due to electronic noise or distortion added to the signal, the amplitude and phase received by the demodulator may differ from the correct value for the symbol. When plotted on a constellation diagram the point representing that received sample may be offset from the correct position for that symbol. For example, a vector signal analyzer can display the constellation diagram of a digital signal by sampling the signal and plotting each received symbol as a point. The result is a “ball” or “cloud” of points surrounding each symbol position. For example, measured constellation diagrams may be used to recognize the type of interference and distortion in a signal.